WHOI-94-1 6

Woods HoleOceanographic

Institution

Altimeter Analysis Of Ocean Currents

by

Sandipa Singh, Michael J. Caruso and Kathryn A. Kelly

* July 1994 <

Funding was provided by the Office of Naval Research under

Contract number N00014-92-J-1486.

Approved for public release; d~stribulion unlimited.

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DISCLAIMEI NOTICE

THIS DOCUMENT IS BESTQUALITY AVAILABLE. THE COPYFURNISHED TO DTIC CONTAINEDA SIGNIFICANT NUMBER OFPAGES WHICH DO NOTREPRODUCE LEGIBLY.

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WHOI-9416

Altimeter Analysis Of Ocean Currents

by

Sandipa Singh, Michael J. Caruso and Kathryn A. Kelly

Woods Hole Oceanographic InstitutionWoods Hole. Massachusetts 02543

July 1994

Technical Report

Funding was provided by the Office of Naval Research underContract number N00014-92-J-1486

Reproduction in whole or in part is permitted for any purpose of the United StatesGovernment This report should be cited as Woods Hole Oceanog Inst Tech Rept.,

WHOI-94 16.

Approved for public release; distribution unlimited.

Approved for Distribution:

Philip L Richardson, ChairDepartment of Physical Oceanography

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AbstractAltimeter data from satellites are being used in an ongoing effort to obtain datasets with temporal as well as global coverage. This report describes the algo-rithms formulated and the programs written for the use of altimeter data fromthe European Space Agency (ESA) European Remote Sensing Satellite, ERS-1,for a repeat track analysis of ocean currents. It also presents some results fromthe California Current region.

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List of Figures1 Orbit tracks for one complete cycle in phase 1 ..... .,.. . 42 Ascending orbit tracks for one complete cycle in phase 2 (1993

data): Northwest Pacific region ....................... 53 ERS-1 ascending subtracks along the California coast . 84 Unfiltered ssh residuals, filtered ash residuals, and geostrophic ve.

locity for 7 profiles along subtrack a149 ................5 RMS ofsahore and onshore velocity for subtrack aOO6 ......... 116 RMS offshore and onshore velocity for subtrack a149 ....... .. 11

iii

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List 0of Tables

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Na ii C.to I{.

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I Introduction

This report describes the use of altimeter data from the'European Space Agency(ESA) European Remote Sensing Satellite, ERS-4,for the analysis of ocean cur-rents. It isthe continuation of the altimeter analysisdeveloped previously byCauso et aL. [1990] for the U.S. Nay altmet satellite, GE0SAT. The commonpurpose of both projects was to perform a 'repeat' or 'collinear' track analysis,which required sorting the data into collineai-tracks, corricting the sea-surfaceheights for various measurement errors, and regridding the along-track-data to acommon grid [!J.

The major difference between the two projects is that the ERS'I satellite hastwo phases. The fist phase consistsof a three-day, repeat orbit and the Secondphase consists of a-35-day repeat orbit. The satellite-remains in ,the first -phase

-for the-first thrce months of. the year, and then switches to "the- second phase-for the rest. of. the year. This- has necessitated the rewriting of several of theGEOSAT data-processing routines so that they can work on the two unique ERS-1orbits. Since the processing is similar for the GEOSAT andERS-1 altimeters, aneffort was made touie as many of the GEOSAT analysis-programs as potsble.This report presents'the similarities and differences in the two data sets-and the

-modifications that were made.to accommodate the ERS-1 data.Section 2 describes the format in which the raw data are stored and the routines

needed to convert it to'the GEOSAT format. -Section 3 describes the orbit paran-eters and characteristics for the two phases of the satellite. Section 4 outlines the,steps required to label and store the data according to phase, cycle and orbit num-bers so as to facilitate repeat track-analysis. It -then analyses the relabeled datafromthe California Current region for the derivation of mean sea surface heights.Section 5 shows some results obtained from the analysis performed in section 4.Finally, section 6 contains descriptions of the various C-program routines.

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Table I:, Geophysical Data Record ContentsItem ERS.1 IGDR I GEOSAT GDR Units Range Bytes

Parameters Parameters1 UTC(Since 11/85) i utc s 28 to 58 42 UTC(contd.) 1 utcm s x10

- 6 0 to 16 43 Latitude lat Degrees x10-

6 = 8.27 44 Longitude Ion Degrees x10-6 0 to 3. 608 45 Orbit orb mm 7.7

s to 8.18 46 Flags (see text) 47 H (uncorrected) m..h cm 1 !.51 48 Sigma(H) s.h cm 0 to 100 29 SWH swh cm0 to 2500 210 Sigma (SWH) s.owh cm 5 to 100 211 Wind Speed (not used) 0.1 m s

" 0 to 350 212 Sigma (Wind Speed) (not used) 0.1 m s - 0 to 35 213 No. in Average (not used) 10 to 20 214 Geold geoid cm = 1.51 215 Solid Tide s-tide mm = 500 216 i Ocean Tide o.tide mm ±: 5000 217 Wet (NMC) tw.fnoc mm 0 to -400 218 Dry (NMC) dfnoc mm -2100 to -2400 219 ono iono mm 0 to -200 220 H Corrn. (Cal) (not used) mm :- 1000 221 Sigma-Naught s-naught 0.01 dB 0 to 4000 222 Int. Electron Dens. (not used) 14 to 21 223 Peaklness (not used) 0.01 100 to 170 224 Wet (SMMR) w.smmr mm 0 to -400 225 Wet SMMI) w.smmi mm 0 to -400 2

3

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200' 210, 221r 230' 245W 250 210

Figure 2: Ascending orbit tracks for one complete cycle in phase 2 (1993 data):Northwest Pacific region

Parameter GEOSAT IERS-1, Phase 1 ERS-1, Phase 2Cycle Period (days) 17 3 - 35Orbital Period (seconds) j6037.55 6027.91 6035.94Orbit Inclination (degrees) j107.98 98.516 98.543Ascending Node (degrees longitude) 356.5878 24.36 20.96Orbits Per ..ycle 244 43 501Reference Date January 1, 1985 January 1, 1985 Janurzy 1, 1985Start Time for 1992 (seconds) 158403168.68 221171732.92 229207416.26Start Time for 1993 (seconds) 252620775.26 265495424.13Spacing between successivedata points (seconds) 0.98 0.98 0.98Intra-rcycle track spacing(degrees Iongitude) 2 0.2

Number of cycles 50 130 18

5

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Table 4: Naming Conventions for record plcmmm.onnn11 number I description range

I phase 1 (3-day)2 (35-day)

mmm cycle 000-029 (for 1=1)000-007 (for 1=2)

o orbit a (ascending)orbit (descendin g)

S nn! orbit 000-042 (for 1=1)__ _ 1000-500 (for 1=2)

4.2 GDR Processing

After the GDRs have been split into individual orbits for the region of interest,they must be filtered, corrected and regridded. The original GEOSAT filteringroutines, gxle=n2 and 9.pike and correction routine, g-corred, were used with-out modification. The splining of the cleaned data to a uniform rectangular gridis carried out using the routine ig.spline (Section 6.4), which is a modified versionof the GEOSAT g. pline routine.

Once all the GDRs are cleaned, corrected and regridded, they can be analyzedusing the modified repeat analysis routine, ig-repeats (Section 6.3). This routinecalculates mean sea surface height from repeat-track data and subtracts it fromeach cycle. Orbital errors are removed by fitting a sine function (one cycle perrevolution), to the residuals and subtracting it.

4.3 Calculation of Geostrophic Velocities

The final stage is to calculate geostrophic velocities, using ash (sea surface height)residuals. The relationship between ash and geostrophic velocity, V 121, is:

-4 6Z

where:g is gravitational force = 9.8ms -2,f, is the Coriolis force 2flsinjj.,6h is ash difference between two successive points on the track (meters), and

7

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the orbital parameters are set and the data is divided into cycle and orbith numbersand labeled according to the conventions described in Table 4. The region maybe specified in two ways, as bounded by latitude lines or bounded by longitudelines. Usage for this program is:

, ig.region dir min..lat maxlat- min..on max..lon [orb.num]

where:dir is I for regions bounded by latitude lines, 2 otherwise;min-lat is minimum latitude, in degrees, for the region to be extracted;max..lat is maximum latitude, in degrees, for the region to be extracted;min.-.on is minimum longitude, in degrees, for the region to be extracted;max.on is maximum longitude, in degrees, for theregion to be extracted; andorb.num are the optional orbit numbers, which are extracted from the region.

6.3 ig-repeats

This program is modified from the GEOSAT program, g.repeats. It needs twoadditional input arguments, the phase and year of the data. It performs a repeattrack analysis of the ERS GDRs and assumes that all GDRs have been cleaned,corrected, and splined to a uniform grid using g-clean, g.correct, g.spike, andig.spline [1]. The program reads in all the available GDRs for a particular trackand calculates the mean sea surface height for each grid point. This mean heightis subtracted from each track to produce a residual height profile. A sine functionis then fit to the residuals as an estimate of the orbit error, and subtracted fromthem to yield further residuals. The usage of the program is:

% ig.'epeats phase year pl/c*/c*.a002c > a002csam

wherephase denotes the phase (I or 2) of the data andyear is the year (92 or 93) of the data.The filename is specified using wildcards so that all cycles associated with thespecified orbit are read in. Thus, in the above example, cycles associated withascending orbit, a002c are read in. The sea surface height mean is written tostdout, a002csm in this example. It contains the following information:

13

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AcknowledgementsThe authors would like to thank John Lillibridge of the National Ocean Serviceat National Oceanic and Atmospheric Administration for supplying the ERS-1altimeter data and corrections, and Barbara Galfron for her suggestions on thetext. Funding for this project was provided by the Office of Naval Research undercontract number N00014-92.J-1486.

15

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5022101REP Ai pOCUENT"IO No"~~ 3. IReciplint' Accession No.RP DCENTTO WI'OI-94-16

Altimeter Analysis Of Ocean Currents July 1994

7. AUUhWrtn Sandipa Singh. Michael J. Caruso and lKathrvn A. Kelly 8 Pesorming Orgerniuatm Rapt. ft.

9. Performig Orprnlitaton Name and Addes 10. Pwpet.'Anlvork Unit No.

Woods Hole Oceanographic Institution 11. ContrntcJc or ermti) NO.WoodS Hole. Mdissachusehts 02543 le M 1-9418

(a)

12. Spooaortog Orgaiirn Nomarnd AtidlessI 3TypeofReort & Perom Cowned

Office of Naval Reeac Technical Report

14,

I& Supplemntry NotesThis report should be cited as. Woods Hole Oceanog. Inst. Tech. Rept., WIHOI.94.16.

16. Abstrac (imlit. 200 word$)

Altimeterdata from satellites are being used in an Ongoing effort to obtain data sets with temporal as wtell at global coverage.Ibis report describes the algonthms formualated and the programs writen for the use of altimeter data from the Furopean SpaceAgency (ESA) European Remotc Sensing Satellite. lERS-1. for a repeat track analysis of ocean currents. Ii alsoA presern iomeresults from the California Current region.

17. Documenot Analysis &. DescriptorsaltinetercdataItRS- ICalifornia current

6. MIoithmrs'Ope.Ended Terms

eCOSATI FietO~rup1I. Aewaautity, Statemeint 1t. secuity CIA" lirle Ropeft) 21. No. of Pagies

Approved for public release: distribution unlitied. as. onCLSonoIhis P0. 24 ic

(84. ANSI-239 IS) See Inainmeins on flevere OPTIONAL FORM 272347)

; es'are of Csffroce

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